A very strong CME could devastate the surface of earth as well by simply overwhelming the magnetosphere. It's happened in the past. One in 1859 produced auroras even at the equator and would have had devastating effects to any electrical grids exposed to it if it happened today. The really bad news is that CME's 10-20 times greater than even that one are possible. (source)

All true. But with no magnetosphere at allany CME would likely be devastating to anyone not in shelter, right? Perhaps even with a thick post terraforming atmosphere. That's the theory at least, but I'd like to know what actually turns out to be the case. We've already heard surprising stuff from Ray R about what Curiosity is picking up without a CME (basically the difference between Mars and Earth could be explained solely by Mars' thin atmosphere).